The transcription factor HoxA10 is expressed early in myelopoiesis and is overexpressed in myeloid leukemia. These observations suggest that differentiation stage specific HoxA10 function may be involved in regulation of myelopoiesis. We noted that repressor elements in the promoters of the genes encoding the respiratory burst oxidase components gp91phox and p67phox (the CYBB and NCF2 genes) are similar to the derived DNA-binding consensus for HoxA10. We found that HoxA10 binds to these cis elements in EMSA with nuclear proteins from undifferentiated, but not WNy-differentiated, myeloid cells. Therefore, HoxA10 DNA-binding decreases as gp91phox and p67phox expression increases. Consistent with this, overexpression of HoxA10 represses gp91p0x and p67phox expression via these CYBB and NCF2 repressor elements. In vitro binding assays indicate that HoxA10 recruits transcriptional co-repressor proteins to the CYBB and NCF2 promoters. We found that IFNy-differentiation of myeloid cell lines results in HoxA10 tyrosine phosphorylation, which decreases HoxA10 DNA-binding affinity. Additionally, we found that SHP1 protein tyrosine phosphatase augments HoxA10 repression of the CYBB and NCF2 genes in undifferentiated myeloid cells. Therefore, we hypothesize that HoxA10 represses myeloid genes early in myelopoiesis and that HoxA10 DNA-binding is regulated by phosphorylation during differentiation. We will investigate this hypothesis by the following specific aims: Aim 1: Determine if HoxA10 repression requires recruitment of histone deacetylase activity to the CYBB and NCF2 genes. If so, does HoxA10 interact with co-repressor proteins directly, or via interaction with Pbxla? Aim 2: Determine if tyrosine phosphorylation during myeloid differentiation decreases the affinity of HoxA10 for the CYBB and NCF2 repressor elements. If so, does SHP1-PTP regulate HoxA10 phosphorylation.? Aim 3: Determine the roles of HoxA10 repression activity and tyrosine phosphorylation in leukemogenesis. These investigations will identify the molecular mechanisms of target gene regulation by HoxA10, which may suggest a role for HoxA10 in differentiation block in myeloid leukemia.